This is no joke, a sprained penis is extremely painful and one may never recover - it's actually more of a break than a sprain. It usually happens when during vigorous thrusting, you miss, and the penis bends in half while erect, rupturing internal blood vessels, and tearing muscle - not pretty at all, so consider it if you start getting carried away.
Put ice on it immediately, take anti-inflamatories, and see a doctor.